^ Salvation lies within.
When my MIL died everyone decided I would go through her book shelf and keep, discard or donate whatever I decided. You would not believe the money I found. I guess it started out as her mad money or something. And she just forgot. We also found cash in envelopes in her underwear drawer and old purses. She was a saver. There were 3 sibs and they split a coupla grand in cash. I squirreled the book cash in my pocket. (Hey, I did the work;))
The economics book by Paul Samuelson, quite possibly the most boring textbook ever written. A thief wouldn’t stand a chance. Just looking at it would make him drowsy, and if he touched it, he’d fall into a comatose sleep.
Intermediate Level Sentence Diagramming for Students of Modern English, Part III (Revised Edition)
Some 102-level Economics textbook.
There’s a serious risk of it being discovered due to not being covered by enough dust, though. It would stand out a bit.
A few years ago there was a piece on NPR about a big book of Victorian Poetry that was very popular at the time, despite not being particularly good. A modern graduate student decided to do his thesis on it, as no one had bothered to before, and discovered the reason for it’s popularity- Hidden in the final third of the book and absent from the table of contents was about a hundred pages of erotica.
I misread this as “Anals of the Great Accountants.” There’s a book you couldn’t pay me to open. :eek:
After my bankruptcy nearly 8 years ago, I was unable to have a bank account and had to deal in cash.
I own a hardcover ‘The Complete Works of William Shakspeare’. During that period of time, I referred to it as "The Complete Bank of William Shakespeare’ because I’d stick my money in it, among different pages so there wasn’t an obvious wad hidden in it. I didn’t figure it was a book people were going to rifle through.
Right next to it on my shelf is ‘Blackadder; the whole damned dynasty’, with all of the Blackadder scripts in it. The episode ‘Money’ was where I put money I needed to set aside for a while. 
Old 1970s-era microwave cookbooks.
For some reason my wife and I both hung on to our foreign language textbooks from high school. I’m going with Second Year Latin. I mean, you have to have experience with at least first-year Latin to even recognize any words, right?
An Introduction to Old Norse, by EV Gordon. That book caused me suffering in grad school, though I will always remember the story of Audun and the bear. (The d is actually eth, the letter that looks like a drunk lower case d that has been crossed.)
Too thin a book. You couldn’t hide much inside it. You need a thick tome that looks too heavy to lift.
Principles of Advanced Auditing Vol. 4.
Let’s get some love for college text books.
Advanced Automated Accounting.
Not only that, you read it starting from the back.
The Rochester, NY yellow pages 1998 edition
744502 was one of the best developed characters in literature.
“Vacation Slides: Wilmington, Delware”
Slight hijack from the OP maybe but --------- my Dad knew he was going to die but he never told the rest of us or our mother. He and I were pretty close and he knew me very well. When I got over the house I went into his den/guest room and started looking over his desk to see what needed attention right away and what we could ignore. My eyes fell right away on a volume about the history of vampires in the folk-traditions of the Rusyns. Well, I have GOT to check this one out. Inside were little post-it notes ---- all they said were things like – the bookcase, my old portable radio, where we fixed the plumbing. Each place was a hide-away with money or valuables and projects I had helped him with or would remember. Some of the things had real value and some just family value that mother may have destroyed (my mother is a little crazy like that) but I would cherish. I can imagine him, having gotten the news from his doctor, saying to himself “when the times comes where is the first place Kopek is going to look ----- what will he just have to pick up that his mother will never look twice at” and then composing cryptic little notes and making them blend in. Dude was a helluva guy and guessed damn well.
Second thing – a friend runs a business that recycles and deals in old books. It isn’t just paper money between the pages; he wands the spines of anything much before 1930 and visually checks almost all the spines. He has a “collection” of coins and currency he has found tucked away there with about 20 nice pieces of gold among them. The thought was that some crooks are going to rifle the pages but who is going to look in the spines? Bibles are one of the better old books to have things hidden there from what he has said.